SECONDARY STRUCTURES OF BARABOO QUARTZITE 269 



THE SECONDARY STRUCTURES INDEPENDENT OE THE FOLD 



It is believed that the joints which are independent of the Baraboo 

 syncline are due to tension rather than compression, since they lack 

 strike and dip relations but are usually vertical, open, and occur 

 in approximately rectangular sets whereas, if they were due to 

 compression, they would occur as closed, inclined shearing joints, 

 striking more nearly in one direction. 



If the joints, which are independent of the synchne, are due to 

 tension, then the most probable source of tensile stresses would be 

 complex folding under conditions of moderate load. Under such 

 conditions folding will produce rectangular sets of strike and dip 

 joints normal to the direction of tensile stress. 



From the description of the secondary structures, it follows that 

 the Baraboo quartzite was subjected to at least one deformation 

 after the folding and before the Cambrian was deposited, and at 

 least one deformation after the deposition of the Cambrian. It 

 seems that one of the principal directions of rupture caused by the 

 deformation following the folding was N. 20-30 W., and therefore 

 it is possible that this was one of the axial directions of this deforma- 

 tion. The directions of maximum rupture in the Cambrian are N. 

 60 E. and N. 30 W., and therefore assuming that these joints are due 

 to tension caused by complex folding, the axes of the Cambrian fold 

 are approximately N. 60 E., and N. 30 W., in the district. The 

 intermediate systems may have originated with the preceding systems 

 of independent joints, or they may have developed through other 

 stresses. 



CONCLUSION 



About 20 per cent, of the joints of the South Range, and 40 per 

 cent, of the joints of the North Range are compressive shearing joints 

 developed in planes parallel to the bedding, and in planes parallel 

 to the strike and diagonal to the bedding by strains developed during 

 the formation of the Baraboo syncline. A part of the joints so 

 classed may be due to tension, but there is no direct evidence of this. 



The remaining joints, constituting about 80 per cent, of the joints 

 of the South Range, and 60 per cent, of the joints of the North Range, 

 are obviously independent of the folding of the range, having no 



